The present invention relates, in general, to control systems for turbomachines; and, in particular, the present invention relates to a control system for an extraction type mechanical drive steam turbine.
In a process plant, waste heat energy may be available for generating steam within a waste heat boiler (steam generator), the output, of which, may be used to drive a steam turbine. The steam output of the waste heat boiler is dependent upon the waste heat energy which is available from a process portion of the plant. It is desirable to utilize all of the waste heat energy available, and the resultant steam flow, while maintaining the boiler pressure and turbine speed at a predetermined operating level. It has been found that in an extraction type steam turbine the boiler operating condition may be maintained by varying the position of the steam inlet valve to control the steam inlet pressure. Moreover, it has been found advantageous to control turbine speed by varying the position of the extraction control valve. If the requirements of inlet pressure control and turbine speed control are satisfied, excess steam flow to the turbine may be extracted to supplement other process requirements.
It is intended, under normal operating conditions, that the inlet valve and the extraction valve be independently positionable to satisfy the foregoing requirements. However, under a condition wherein the extraction valve is completely closed and the turbine speed continues to rise, then it becomes necessary for the speed control to override the turbine inlet pressure control and thereby regulate turbine speed by using the inlet valve.
It is therefore one object of the invention to provide a control system for an extraction type steam turbine wherein turbine inlet steam pressure (flow) and turbine speed may be controlled independently of one another.
It is another object of the present invention to provide a control system for an extraction type steam turbine wherein, under certain conditions, the speed control overrides the inlet pressure control.
In order to satisfy the foregoing objects and requirements, an extraction type steam turbine is provided with a control system according to the present invention. In general, steam sufficient to satisfy turbine load and speed requirements is provided by a waste heat boiler, the output of which is input into the steam turbine through a steam inlet valve. The inlet valve is positioned according to a pressure or flow signal indirectly indicative of boiler operating condition. The turbine speed is controlled by an extraction control valve which is positioned according to a turbine speed signal. After the turbine inlet pressure requirement and turbine speed requirement have been satisfied, the excess steam may be extracted from the turbine to supplement other process plant requirements. If turbine speed can no longer be controlled by means of the extraction valve, the inlet valve position signal may be overridden by the speed control signal through an inlet valve positioning device until such time as normal speed control can be resumed.
The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood with reference to the following description, taken in connection with the drawing.